The present invention relates to a high margin notification method and apparatus, particularly for use in a mobile communication system.
Terrestrial cellular communication systems are well-known in the art and a number of standards exist which define different signalling protocols for them, such as the GSM standard and the PCN standard. Details of these standards are given for example in xe2x80x9cMobile Radio Technologyxe2x80x9d by Gordon White, Butterworth Heinemann, 1994. The GSM and PCN systems are digital communication systems and allow paging and data communications in addition to voice communications. In the GSM system, a short message service (SMS) is provided for transmitting short messages to mobile terminals.
Mobile satellite communication systems have also been proposed in which radio frequency links are provided by a constellation of satellites. These satellite communication systems provide much greater coverage than terrestrial cellular systems. One example of such a system is the ICO(trademark) system, aspects of which are described in patent publications WO 95/28747, WO 96/03814 and GB 2,295,296A. Other examples include the Inmarsat(trademark) satellite system as described in xe2x80x9cSatellite Communications: Principles and Applicationsxe2x80x9d by Calcutt and Tetley, published 1994 by Edward Arnold, the Iridium(trademark) satellite cellular system, described for example in EP-A-0365885, and the Odyssey(trademark) system described for example in EP-A-0510789, EP-A-0575678 and EP-A-0648027.
However, in both terrestrial and satellite mobile communication systems, it is not always possible to communicate with a mobile user, since the received signal strength at the mobile user terminal may be severely attenuated, for example by blockage or multi-path fading. Furthermore, the gain of the user terminal may be low, for example because the user terminal antenna is retracted or not deployed in the optimum position.
A satellite paging system is disclosed in U.S. Pat. No. 5,392,451 in which, if a call receiver does not acknowledge receipt of a paging message, the paging signal is repeated with a higher transmission power.
A paging system is disclosed in WO96/08941 in which, if a mobile telephone does not respond to a paging signal, the paging signal is sent on a different channel, with more error correction or higher power if necessary.
However, the transmission power of a satellite is limited by the power supply available, such as the output power of solar cells of the satellite or by the battery power of the satellite during an eclipse.
According to one aspect of the present invention, there is provided a method and an apparatus in which, if a message is not acknowledged by a mobile terminal, a different message, such as a shorter message, is transmitted to the mobile terminal with a higher margin. The different message may be selected so as to minimise the total energy needed to transmit it, whilst still sending sufficient meaningful data to the mobile terminal. In this way, the total transmit power required to provide such a service may be kept below a predetermined limit.
In accordance with another aspect of the present invention, there is provided a method and apparatus for registering a mobile terminal with a communication network, in which the mobile terminal receives a high penetration signal from the network and waits until it is in range of a lower penetration signal before sending a registration signal to the network so that information about the mobile terminal can be updated by the network. In this way, registration is initiated by means of a high penetration signal, thus increasing the probability of the mobile terminal receiving this signal, but registration is completed by means of lower penetration signals, thereby reducing the need for transmission of high penetration signals.